It is well known in the art that the resistance modulation of magnetoresistors can be employed in position and speed sensors with respect to moving magnetic materials or objects (see for example U.S. Pat. Nos. 4,835,467, 4,926,122, and 4,939,456). In such applications, the magnetoresistor (MR) is biased with a magnetic field and electrically excited, typically, with a constant current source or a constant voltage source. A magnetic (i.e., ferromagnetic) object rotating relative and in close proximity to the MR, such as a toothed wheel, produces a varying magnetic flux density through the MR, which, in turn, varies the resistance of the MR. The MR will have a higher magnetic flux density and a higher resistance when a tooth of the rotating target wheel is adjacent to the MR than when a slot of the rotating target wheel is adjacent to the MR. The use of a constant current excitation source provides an output voltage from the MR that varies as the resistance of the MR varies.
Increasingly more sophisticated spark timing and emission controls introduced the need for crankshaft sensors capable of providing precise position information during cranking. Various combinations of magnetoresistors and single and dual track toothed or slotted wheels (also known as encoder wheels and target wheels) have been used to obtain this information (see for example U.S. Pat. Nos. 5,570,016, 5,714,883, 5,731,702, and 5,754,042).
An example of a known sensor used with a target wheel is depicted at FIGS. 1A and 1B. The sensor 10 consists of a first magnetoresistor MR1, a second magnetoresistor MR2, a biasing magnet 16, a first resistor R1, a second resistor R2, and terminals 22, 24, 26, and 28. The positive terminal 30 of voltage source +V is applied to terminal 22 and the negative terminal 32, considered to be at ground, is applied to terminal 28. As a result, a voltage V.sub.MR appears at terminal 24 which is produced by the voltage divider circuit of MR1 and MR2, and a voltage V.sub.REF appears at terminal 26 which is produced by the voltage divider circuit of R1 and R2. V.sub.MR is input to the non-inverting terminal of a comparator (with hysteresis) 34 and V.sub.REF is input to the inverting terminal of the comparator, wherein the comparator is supplied with power through voltage source +V which is applied to terminal 36 and ground applied to terminal 38.
As shown at FIG. 1B, the sensor 10' may optionally include the comparator 34, whereupon the only outputs therefrom are terminal 30 for the voltage source +V, terminal 40 for V.sub.OUT and terminal 32 for ground.
According to the prior art, the following method is used to match MR1 with MR2. During the manufacturing process of the sensor 10, R1 and R2 do not have the same value to ensure that V.sub.OUT 40 of comparator 34 is at a high voltage level (or, if desired, a low voltage level). V.sub.REF is gradually changed by trimming of R1 or R2 until V.sub.OUT switches voltage levels. For example, trimming is performed by a laser beam 42 of a laser 44, wherein a portion of the cross-section 46, 46' of the selected resistor is ablated to thereby change its resistance.
Instead of the reference signal being adjusted, the DC offset of the input signal (V.sub.MR) can be adjusted to achieve the same goal. FIG. 1A depicts a sensor 10 having an optional known DC offset 52 therefor incorporated therein, wherein the active path of the two alternative paths shown by dashed lines in FIG. 1A depends on whether the DC offset is present. FIG. 1C depicts an example of a DC offset 52 in the form of a variable resistance R3, as for example a rheostat (potentiometer) or a resistor selector box having a plurality of selectable resistors. Alternatively, the DC offset can be effected by individually adjustable current sources.
According to this method of the prior art, the reference signal or the DC offset is adjusted to match the MRs in free space. Now, the sensor 10 undergoes final packaging. Thus, this manufacturing process eliminates MR and bias magnet mismatch.
However, accurate engine crank position information is needed for ignition timing and state and federally mandated misfire detection. The crank position information is encoded on a rotating target wheel in the form of teeth and slots. The edges of the teeth define predetermined crank positions. The sensor is required to detect these edges accurately and repeatably over a range of air gaps and temperatures. Preferably, the output signal of the sensor should indicate a tooth edge passing through the nominal centerline of the sensor, although, a small fixed offset is acceptable. Usually, the specified accuracy is plus or minus 0.5 degrees with respect to the actual edge, which provides a one degree tolerance band. Quite frequently, however, fundamentally good sensors with even tighter tolerance bands must be rejected because they do not fall into the plus or minus 0.5 degree range, e.g. a sensor having a tolerance band from 0.25 degrees to 0.75 degrees.
Accordingly, what is needed in the art is a method to adjust a sensor so that its output signal will indicate a tooth edge location coinciding with the nominal center line of the sensor, or coinciding with any other specified point on the face of the sensor.